Detailed evaluation of electric demand load shifting potential of heat pump water heaters in a hot humid climate

Abstract

AbstractHeat pump water heaters (HPWH) are a proven method of reducing water heating energy use over prevailing electric resistance systems (ERWH). Both technologies lend themselves to enhanced control for peak load reduction. Laboratory tests were conducted in Central Florida using the CTA-2045 standard to evaluate load shifting strategies with connected water heaters. Four HPWHs from three manufacturers, including two different tank volumes were tested alongside an ERWH in a garage-like environment. Tests aimed to shift energy use away from utility peak load periods to off-peak times when excess renewable energy resources are available. Two load-shifting strategies were shown effective, Shed and Critical Peak, with variation by manufacturer. Beyond draw volume, other factors influenced HPWH load shifting:Florida winter conditions, which increase the energy used per draw, provided the greatest challenges to complete load shift. Inlet water temperature had a large impact on the success of load reduction. Ground temperatures in which water pipes were buried largely determined inlet water temperatures.HPWH efficiency setting: Heat pump water heaters often default to a “hybrid” mode that may use some electric resistance heat to minimize risk of running out of hot water. Operational mode can impact load shifting potential. BackgroundHeat pump water heaters (HPWH) are a well demonstrated technology to significantly reduce electricity consumption for meeting household hot water needs. A variety of monitored projects around the U.S. have shown savings of 50-70%, reflected by operational coefficient of performance (COP), relative to conventional electric resistance storage water heaters (Colon et al. 2016; Shapiro and Puttagunta 2016; Willem, Lin, and Lekov 2017). Within the last decade, systems have shown even higher operational COPs from improved compressors and other design enhancements. (Willem, Lin, and Lekov 2017).Beyond the ability to save water heating electricity, HPWHs can also cut peak demand. Many large utility providers in the southeast already have demand response and load management programs (Butzbaugh and Winiarski 2020) and may find value in promoting grid-connected HPWHs capable of load shifting if demonstrated to provide superior load control. This can be thought of as the ability to not only control utility-coincident peak loads, but also to alter the water heating electrical demand profile in a significant manner (e.g., alter electric load profile shape to consume a greater amount of daytime utility scale renewable energy). Current HPWHs and some ERWHs available for purchase are compatible with CTA-2045-A protocol (ANSI/CTA 2018). This protocol has demonstrated electric demand flexibility in the Northwest to provide a utility the ability to control when an appliance draws power from the grid (Metzger et al. 2018). And Carew et al (2018) have detailed simulation studies of load shifting with HPWHs. Other related work evaluating HPWHs has been conducted around California’s Title 24 standard development (Hendron et al. 2020). A multifamily load shifting HPWH study completed detailed modelling that showed higher annual kWh use (11-18%), but averaged a 68% reduction in on-peak energy in a field monitor study in a Northern California climate (Hoeschele and Haile 2022). However, this is the first evaluation to undertake comprehensive laboratory testing.The CTA-2045 protocol standardizes both the hardware interface between a communications module and ‘smart’ appliance, as well as the language used by electricity providers to communicate with a connected device. Manufacturers determine how water heaters respond to the control commands, based on engineering parameters and the water temperature profile in the tank, and thus differences can exist in implementation of the protocol.DisclaimerAs a service to authors and researchers we are providing this version of an accepted manuscript (AM). Copyediting, typesetting, and review of the resulting proofs will be undertaken on this manuscript before final publication of the Version of Record (VoR). During production and pre-press, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal relate to these versions also. Notes1 Test conditions for HPWHs in the UEF test procedure are 67.5 °F (19.7 °C) dry bulb air temperature (+/- 2.5 °F (1.4 °C)) and 50% Relative Humidity (+/- 2), and inlet water temperature of 58 °F (14.4 °C) (+/- 2 °F (1.1 °C)).